Hydrogels offer excellent biocompatibility and have been shown to have reduced tendency for inducing thrombosis, encrustation and inflammation when used in medical devices. Unfortunately, the use of hydrogels in biomedical device applications has been hindered by poor mechanical performance. Many medical devices use hydrogels to improve device biocompatibility, however, the hydrogel can only be used in coatings as a result of insufficient mechanical performance for use as a bulk polymer. Hydrogels suffer from low modulus, low yield stress and low strength when compared to non-swollen polymer systems. Lower mechanical properties result from the swollen nature of hydrogels and the nonstress bearing nature of the swelling agent.
Polyols are well known plasticizers for polysaccharide dry films and are typically listed in the supplier literature as a preferred plasticizer. However, a plasticizer functions to lower modulus, lower yield stress, and often lowers strength.
U.S. Pat. No. 2,426,125 discloses the reaction between alginic acid in the form of threads, fibers or particles and certain lower epoxy paraffins or alkylene oxides to produce glycol alginates.
U.S. Pat. No. 3,640,741 discloses compositions which use polyols including glycerol as crosslinking agents for manufacture of slow dissolving plastic compositions. Excess polyol may be included as liquid media for carrying out the crosslinking reaction. Hydrophilic colloids such as carboxymethyl cellulose gum or alginate gum are crosslinked with the polyol to form a gel which exhibits slow dissolution in aqueous media. This reference does not disclose the formation of stable gels in the presence of water.
There remains the need to provide a means for temporarily reducing the swelling of hydrogel-containing medical devices to a controlled degree, thereby increasing modulus, yield stress and strength, while maintaining flexibility and the ability to return to the nascent hydrogel state once the device is placed in vivo. There also is the need for medical devices comprising hydrogels which can withstand aggressive physician handling without damage, sustain higher stresses encountered during implantation or insertion without inelastic deformation while retaining the original hydrogel properties in vivo.